This invention relates to a hybrid or compound antenna unit and, in particular, to a hybrid antenna unit for mounting a plurality of different antennas therein.
In the manner which is known in the art, recently, various different antennas are mounted in a vehicle such as an automobile. For example, there are, as such antennas, a GPS (Global Positioning System) antenna, an SDARS (Satellite Digital Audio Radio Service) antenna, a radio telephone antenna, an AM/FM radio antenna, or the like.
The GPS (Global Positioning System) is a satellite positioning system using artificial satellites which are called GPS satellites. The GPS is a system which receives radio waves (GPS signals) from four GPS satellites among twenty-four GPS satellites orbiting the Earth, measures, on the basis of the received waves, a position relationship and a time error between a mobile object and the four GPS satellites, and accurately calculates, on the basis of a principal of triangulation techniques, a position and/or a height of the mobile object on a map.
In recent years, the GPS is used in a car navigation system for detecting a position of a running automobile or the like and becomes widespread. In the car navigation system, a car navigation apparatus comprises a GPS antenna for receiving the GPS signals, a processing unit for processing the GPS signal received by the GPS antenna to detect a current position of the vehicle, a display unit for displaying, on the map, the position detected by the processing unit, and so on.
On the other hand, a rod antenna (a pole antenna) is known as a 3-waveband antenna which is capable of receiving three radio waves of portable radiophone bands, FM radio bands, and AM bands. In addition, a multi-frequency antenna capable of receiving four radio waves of the portable radiophone bands, the FM radio bands, the AM radio bands, and GPS radio bands is proposed in EP0862239 (B1) which will be called a patent document 1 hereinafter. The multi-frequency antenna disclosed in the patent document 1 comprises an antenna element, a base, a circuit board affixed on the base in a standing condition, and a GPS antenna. A GPS antenna storage part for mounting the GPS antenna is integrally formed in the base so that it is orthogonal to the circuit board. The antenna element is inclined from a vertical position on the base. With this structure, the antenna element does not have a detrimental affect on the GPS antenna although the GPS radio waves arrive at a low angle of elevation.
In addition, a dual antenna is disclosed in U.S. Pat. No. 6,879,294 issued to Dou Yuanzhu that will be called a patent document 2 hereinafter. The dual antenna disclosed in the patent document 2 is unitized by installing in parallel a patch antenna capable of transmitting and receiving a circularly polarized electromagnetic wave to and from the GPS satellite or the like and a rod-shaped antenna for transmitting and receiving a linearly polarized electromagnetic wave for use in mobile telephony or the like. In the dual antenna disclosed in the patent document 2, a direction of a power feed patch relative to the rod-shaped antenna is set so that a short axis thereof substantially orthogonally crosses a plane containing an intersection between the short axis and a long axis of the power feed patch of the patch antenna and an axis of the rod-shaped antenna. With this structure, it is possible to make less susceptible to the adverse effect of the electromagnetic wave radiated from the rod-shaped antenna on the patch antenna adjoining the rod-shaped antenna.
Furthermore, another antenna unit suitable to insert in a car body is proposed in Japanese Unexamined Patent Publication Tokkai No. Hei 10-107542, namely, JP 10-107542 A1 which will be called a patent document 3 hereinafter. The antenna unit disclosed in the patent document 3 can receive both of a radio wave signal radiated from an artificial satellite and a terrestrial wave signal radiated from a terrestrial antenna. In the antenna unit disclosed in the patent document 3, a planar antenna is mounted on a main surface of a board provided in a case. The planar antenna has an upper side directed upward. An antenna element acting as a helical antenna has a base end displaced from the planar antenna toward a side and has a tip provided with a tilt of about 30 degrees from a perpendicular line in the displaced direction.
In addition, a three-wave receiving antenna apparatus is proposed in EP0747993 (A1) which will be called a patent document 4 hereinafter. The three-wave receiving antenna apparatus disclosed in the patent document 4 is an integrated antenna apparatus comprising an AM/FM wave receiving antenna capable of receiving an AM wave and an FM wave and a GPS wave receiving antenna capable of a GPS wave transmitted via a GPS satellite. The integrated antenna apparatus is suitable for, e.g., a vehicle antenna apparatus.
On the other hand, the SDARS (Satellite Digital Audio Radio Service) is a radio service according to a digital radio broadcasting using artificial satellites (which will called “SDARS satellites” hereinafter) in the United States of America. That is, in recent years, a digital radio receiver, which receives the satellite wave from the SDARS satellites or the terrestrial wave so as to listen to the digital radio broadcasting, has been developed and is put to practical use in the United States of America. Specifically, two broadcasting stations called XM and Sirius provide radio programs on 250 or more channels in total. The digital radio receiver is generally mounted on a mobile object such as an automobile and is adapted to receive a radio wave having a frequency of about 2.3 gigahertz (GHz) as a received wave to listen to the digital radio broadcasting. In other words, the digital radio receiver is a radio receiver capable of listening to mobile broadcasting. Inasmuch as the received wave has the frequency of about 2.3 GHz, a reception wavelength (resonance frequency) A thereof is equal to about 128.3 mm. It is noted here that the terrestrial wave is a radio wave obtained by receiving the satellite wave at a ground station, slightly shifting the frequency of the satellite wave, and retransmitting the linear polarized wave. Thus, the terrestrial wave is the linear polarized wave exhibiting linear polarization while the satellite wave is a circular polarized wave exhibiting circular polarization.
An XM satellite radio antenna apparatus normally serves to receive circular polarized radio waves from two stationary satellites and, in an insensitive zone of the circular polarized waves, receives a radio wave by using a terrestrial linear polarization portion of the radio antenna apparatus. On the other hand, a Sirius satellite radio antenna apparatus normally serves to receive circular polarized radio waves from three orbiting satellites (synchronous type) and, in the insensitive zone, receives a radio wave by a terrestrial linear polarization portion of the radio antenna apparatus.
As described above, the radio wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting. Therefore, an antenna for receiving the radio wave must be located outside as known in the art. If the digital radio receiver is mounted in the mobile object such as the automobile, the antenna unit is must be attached to a roof of the mobile object (car body).
In the manner which is described above, various types of hybrid antenna units for mounting a plurality of antennas of different types therein have been known in the art. In addition, those skilled in the art might easily come with ideas using, as one of the hybrid antenna units, one mounting a GPS antenna and a SDASR antenna on a main surface of a circuit board as well as the above-mentioned 3-waveband antenna capable of receiving the three radio waves of the portable radiophone bands, the FM radio bands, and the AM bands thereon. Under the circumstances, if a booster circuit for the AM/FM radio bands is mounted on the main surface of the circuit board, circuit parts constituting the booster circuit serves as an impediment or obstruction (blind) with respect to the GPS antenna and/or the SDARS antenna.
Accordingly, it is necessary for the last-mentioned hybrid antenna unit to mount the booster circuit on the circuit board at a suitable position thereof.